1. Field of the Invention
This invention relates broadly to electrolytically treating a selected cylindrical surface of an article. More particularly, the invention relates to controlling the extent of electrolytic action on each portion along a cross section of the surface with respect to the other portions along the cross section. Consequently, the invention is useful in controlling the amount of gold which is selectively deposited over a formed spring of a connector contact.
2. Discussion of the Prior Art
In present day electronic equipment gold is widely used as a surface material on contacts. The excellent conductive properties of gold, and its extreme resistance to corrosion, are highly desirable characteristics of contact surfaces. However, the high cost of gold makes it advantageous to find various ways of limiting its use to precisely defined areas and to attempt to limit its deposits to thicknesses no greater than those necessary to achieve such desirable characteristics. While the present invention relates to plating controlled thicknesses of material onto selected surfaces of articles, such as connector contacts, it must be understood that the description of the invention in relation to such contacts is for illustrative purposes only.
In the prior art, as exemplified by U.S. Pat. No. 3,891,534 to Cordone et al., electrolytic plating systems have been used, for example, to cathodically plate an epitrochoidally-shaped internal surface of a rotary engine housing. An internal cylindrical surface appears to be uniformly platable by a concentric conforming anode. On the other hand, a noncircular cylindrical internal surface, such as one having a reverse curvature, needs a variable anode-to-cathode spacing for a uniform plating thickness over the internal surface of the article. The Cordone et al. patent addresses itself to plating a specific shape of an epitrochoid or compound cathode surface with a reverse curvature. The Cordone et al. disclosure teaches that for the portion of reverse curvature of the cathode, the anode-to-cathode distance is progressively deviating away from conformity and having a maximum deviation in alignment with an extension of a radius passing through the mid point of said reverse curvature. The deviation is taught to be inversely proportional to the square root of the current density.
It is also known to selectively plate areas of a substrate surface with an impinging stream of electrolyte. The method includes the use of a dielectric member maintained between an anode and a cathode which is out of contact with the latter.
It is desirable to improve on the prior art, particularly to improve on the uniformity of the material which is deposited within the selected surface area of the workpiece to be plated. It is further desirable to achieve such uniformity of material deposition in a predetermined area of the workpiece regardless of the amount of curvature in the particular area and independently of the current density at which the workpiece is to be plated.